1. Field of the Invention
The present invention relates to a magnetic sensor device, and more particularly, to a magnetic sensor device which has an orthogonal-type thin film magnetoresistive element (hereinafter referred to as an MR element) and which is used for the angle detection, position detection and the like. The invention also relates to a method of manufacturing the same device.
2. Description of the Related Art
FIG. 10 is a top view illustrating one example of a known magnetic sensor device. Referring to FIG. 10, an MR element 1, a semiconductor device (hereinafter referred to as an IC chip) 2 and a multilayer capacitor 5 are mounted on a ceramic circuit board 3 and a thick film resistor 4 and wiring (not shown) are further printed, thus forming a hybrid integrated circuit on the circuit board 3. A plurality of leads 6 are also mounted on the circuit board 3. The circuit board 3 is inserted to a frame 8 equipped with a magnet 7, thereby completing the assembly. The magnet 7 is a substantially disk shaped member and a coaxial rotary shaft 7a is attached to the bottom of the magnet 7, thereby rotatably supporting the magnet 7 to the frame 8.
The MR element 1 made of a ferromagnetic substance, such as Ni, Co and Fe, is formed in a strip-like shape on the circuit board 3. FIG. 11 is a block diagram of the MR element 1. As shown in FIG. 11, in the MR element 1, two ferromagnetic resistors 1a and 1b used as a sensor portion are arranged at right angles to each other and connected in series at the junction B. Such an MR element 1 is referred to as an orthogonal-type element in which the currents flowing in the respective ferromagnetic resistors 1a and 1b meet at right angles. The ferromagnetic resistors 1a and 1b have the same resistance. Thin films may be formed in an elongated bar-like shape as both ferromagnetic resistors 1a and 1b, but instead, as illustrated, for example, in FIG. 12, they also may be formed in a zigzag shape in order to highly increase the impedance of the MR element 1 and also to downsize the MR element 1. In the latter case, the ferromagnetic resistors 1a and 1b are formed symmetrical with respect to the phantom line A--A. The currents flowing in any portions symmetrical to each other in the respective ferromagnetic resistors 1a and 1b also intersect at right angles.
When a current flows into the MR element 1 and the magnet 7 is rotated, the angle between the current flowing in the MR element 1 and the magnetic field generated by the magnet 7 varies in accordance with the rotation of the magnet 7, thereby continuously changing the resistance of the MR element 1. This is called the magnetoresistance effect. By making use of this effect, the magnetic sensor device equipped with the MR element 1 illustrated in FIG. 10 converts a variance in the angle between the current and the magnetic field into a change in the voltage, thereby performing angle and position detection. The rotary shaft 7a provided for the magnet 7 is operationally connected for cooperation with the rotary shaft of a rotating member to be detected (not shown). The magnet 7 is rotated in accordance with the rotation of the rotary shaft of the rotating member, thereby changing the angle of the magnetic field applied to the MR element 1. Thus, the resistance of the MR element 1 is also varied due to the above magnetoresistance effect, and the output voltage of the overall hybrid integrated circuit is accordingly varied. As a result, the rotation angle of the rotating member can be detected. Such a magnetic sensor device is now widely used for automobiles, VTR motors, office automation devices, factory automation devices, etc.
The description will now be given of a method of manufacturing the magnetic sensor device. The hybrid integrated circuit illustrated in FIG. 10 is first formed on the circuit board 3. Then, a thick film resistor 4 in the hybrid integrated circuit is trimmed to adjust output characteristics of the hybrid integrated circuit. A method of adjustment is employed whereby the thick-film resistor 4 is cut by a laser beam, or the like, thereby gradually varying the resistance of the thick-film resistor 4 so that the output of the overall hybrid integrated circuit reaches a predetermined reference voltage. Subsequently, the leads 6 are arranged on the circuit board 3 which is then assembled into the frame 8 equipped with the magnet 7.
As stated above, in the known magnetic sensor device, the thick film resistor 4 is trimmed without applying the magnetic field to the MR element 1 before the circuit board 3 is assembled into the frame 8, thus adjusting the output characteristics of the hybrid integrated circuit on the circuit board 3. As described above, the orthogonal-type MR element 1 shown in FIG. 11 is formed in such a way that the ferromagnetic resistors 1a and 1b have the same resistance when the magnetic field is not applied, i.e., R1=R2. After the circuit board 3 is assembled into the frame 8, however, the resistance values of both ferromagnetic resistors 1a and 1b differ from each other due to a change in the angle of the magnetic field generated by the magnet 7. As indicated by the arrows shown in FIG. 11, when the horizontal magnetic field is accurately applied to the MR element 1, the angle between the magnetic field and the current flowing in the resistors 1a and 1b is constant, thus equalizing .the resistance of both resistors 1a and 1b. That is, in the cases where the horizontal magnetic field is applied and where the magnetic field is not applied at all, the voltage across A and B is equal to that across B and C, and thus, if the same power voltage supplied to the two resistors 1a and 1b, the same output voltage should be obtained. However, if the MR element 1 slipped out of place or is misaligned with respect to the circuit board 3 when it was mounted, or if the circuit board 3 slipped out of place or is misaligned with respect to the frame 8, the angle between the MR element 1 and the magnetic field generated by the magnet 7 becomes changed from the proper one. Therefore, the output voltage gained by applying the horizontal magnetic field to the MR element 1 after assembly does not match the reference voltage adjusted during trimming in the absence of the magnetic field, thus lowering the precision of the output characteristics, further resulting in a failure in correct detection as a sensor. It is also very difficult to trim the thick film resistor 4 on the substrate 3 after assembly.